My bet is on Hulkenberg.

As odd as it sounds, Sainz is too good. RBR doesn't need another driver who can challenge for the win. That only creates friction. That means that Sainz is not a good option in my opinion.

Ricciardo is apparently too slow when driving the sim, much slower than Perez.

I think Hulk is decent enough, but also realistic enough not to start drama within the team.